The present invention relates generally to packaging of fiber optic components. More specifically, the present invention relates to methods and apparatus for packaging fiber gratings, filters, and other fiber optic components to provide a variety of improved features, such as athermalization, support, or protection.
A periodic variation in refractive index of the waveguide along the long axis of the waveguide is commonly known as an optical waveguide grating. A fiber Bragg grating is an optical waveguide grating in a waveguide fiber which will selectively filter propagated light having a wavelength which is twice the period of the grating. Such a fiber Bragg grating is useful as a wavelength filter.
Fiber Bragg gratings are particularly sensitive to temperature changes which, through thermal expansion of the waveguide fiber, cause changes in the refractive index of the waveguide fiber. Changes in grating spacing and changes in the refractive index with temperature variations cause wavelength shifts in the device.
For many applications, fiber gratings must operate over large temperature ranges with minimal change in spectral properties. While the peak loss of the grating will change with temperature, the primary effect of a temperature change is a shift in peak wavelength. This temperature dependence can be compensated for by attaching the fiber grating to a substrate with a negative coefficient of thermal expansion. In one approach, fiber gratings are athermalized, or temperature compensated, by attaching them to a small bar of xcex2 eucryptite, a ceramic substrate with a negative coefficient of thermal expansion (CTE). A frit of at least two compositions attaches the optical fiber to the substrate and an epoxy deposit provides strain relief. The fiber grating attached to the substrate is then typically embedded in a protective fluorogel coating and enclosed in an hermetically sealed metal box to provide protection from the effects of humidity. This design depends upon integral bonding of the frit to a flat surface with a mismatched CTE. Stresses are created at the interface between the flat surface and the frit. This design is asymmetrical, leading to asymmetric forces acting on the optical fiber during thermal cycling. The manufacture of this package involves a large number of process steps and involves a labor intensive process.
Accordingly, it would be highly advantageous to provide a passive temperature compensating package assembly for fiber gratings which provides symmetrical packaging, ease of manufacturing, increased reliability, or a single frit composition.
The present invention provides advantageous methods and apparatus for packaging fiber gratings and other fiber optic components to provide a variety of improved features, such as athermalization, support, or protection. According to one aspect of the invention, a hollow tube having a negative CTE is employed to form an athermalized hollow tube package. The hollow tube surrounds an optical fiber containing a fiber grating, and is contained within a cylindrical body with end caps.
According to another aspect of the invention, a slotted hollow tube having a negative CTE with a longitudinal slot is employed to form an athermalized slotted hollow tube package. The slotted hollow tube surrounds an optical fiber containing a fiber grating, and is contained within a cylindrical body with end caps. The slotted hollow tube allows the fiber grating to be written in the optical fiber after the optical fiber is placed within the slotted hollow tube.
According to another aspect of the invention, the end caps fit within the ends of the cylindrical body.
A more complete understanding of the present invention, as well as further features and advantages of the invention, will be apparent from the following detailed description and the accompanying drawings.